Contents

It will help us to appreciate the astronomical significance of what we have learnt in the previous lecture if
we turn from the general to the particular and see how it applies to individual stars. I will take two stars
round which centre stories of special interest, and relate the history of our knowledge of them.

The Story of Algol

This is a detective story, which we might call 'The Missing Word and the False Clue'.

In astronomy, unlike many sciences, we cannot handle and probe the objects of our study; we have to
wait passively and receive and decode the messages that they send to us. The whole of our information
about the stars comes to us along rays of light; we watch and try to understand their signals. There are
some stars which seem to be sending us a regular series of dots and dashes -- like the intermittent light
from a lighthouse. We cannot translate this as a morse code; nevertheless, by careful measurement
we disentangle a great deal of information from the messages. The star Algol is the most famous of
these 'variable stars'. We learn from the signals that it is really two stars revolving round each other.
Sometimes the brighter of the two stars is hidden, giving a deep eclipse or 'dash'; sometimes the faint
star is hidden, giving a 'dot'. This recurs in a period of 2 days 21 hours -- the period of revolution of the
two stars.

There was a great deal more information in the message, but it was rather tantalizing. There was, so
to speak, just one word missing. If we could supply that word the message would give full and accurate
particulars as to the size of the system -- the diameters and masses of the two components, their absolute
brightness, the distance between them, their distance from the sun. Lacking the word the message told
us nothing really definite about any of these things.

In these circumstances astronomers would scarcely have been human if they had not tried to guess
the missing word. The word should have told us how much bigger the bright star was than the fainter,
that is to say, the ratio of the masses of the two stars . Some of the less famous variable stars give us
complete messages. (These could accordingly be used for testing the relation of mass and absolute
brightness, and are represented by triangles in Fig. 7.) The difficulty about Algol arose from the excessive
brightness of the bright component which swamped and made illegible the more delicate signals from
the faint component. From the other systems we could find the most usual value of the mass ratio, and
base on that a guess as to its probable value for Algol. Different authorities preferred slightly different
estimates, but the general judgement was that in systems like Algol the bright component is twice as
massive as the faint component. And so the missing word was assumed to be 'two'; on this assumption
the various dimensions of the system were worked out and came to be generally accepted as near the
truth. [Note...Rougher estimates were made much earlier...]That was sixteen years ago.

In this way the sense of the message was made out to be that the brighter star had a radius of 1,100,000
kilometres (one and a half times the sun's radius), that it had half the mass of the sun, and thirty times
the sun's light power, &c. It will be seen at once that this will not fit our curve in Fig. 7; a star of half the
sun's mass ought to be very much fainter than the sun. It was rather disconcerting to find so famous
a star protesting against the theory; but after all the theory is to be tested by comparison with facts and
not with guesses, and the theory might well have a sounder basis than the conjecture as to the missing